1. Field of the Invention
Aspects of the present invention relate to an image forming apparatus.
2. Description of the Related Art
Generally, an electrophotographic image forming apparatus is configured such that a light beam is radiated onto a photosensitive member charged with an electric potential, to form an electrostatic latent image on the surface of the photosensitive member. The electrostatic latent image is developed into a visible image, by adhering toner to the electrostatic latent image, and the visible image is transferred and fixed onto a printing medium. The electrophotographic image forming apparatus can be classified as a mono-image forming apparatus, if it can print only a black and white image, or as a color image forming apparatus, if it can print a color image.
The black and white image forming apparatus and the color image forming apparatus commonly include: a developing unit having a photosensitive member on which an image is formed; a developer supply device, which adheres a developer (toner) to the photosensitive member; an exposure unit, which radiates a light beam to the photosensitive member according to an image signal, to form an electrostatic latent image; a charge unit, which charges the surface of the photosensitive member with a predetermined electric potential; a transfer unit, which transfers a visible image formed by the developing unit onto a printing medium; and a fixing unit, which applies heat and pressure to the printing medium, onto which the visible image is transferred, to fix the visible image to the surface of the printing medium.
The color image forming apparatus is provided with: four developing units, which respectively form yellow, magenta, cyan, and black images; four exposure units, which radiate a light beam onto the respective developing units; and four charge units, which charge the photosensitive members of the respective developing units.
An image forming process of the color image forming apparatus will now be described. First, the surface of the respective photosensitive members is charged with a predetermined electric potential by the charge units. When the respective exposure units radiate a light beam onto the surface of the charged photosensitive members, electrostatic latent images are formed on the surface of the photosensitive members by an electric potential difference. By adhering developers corresponding to the four colors to the electrostatic latent images, visible images of the four colors are formed. The visible images are transferred to an intermediate transfer belt, or the printing medium, on top of one another, by the transfer unit, and thus a color image is formed.
When the printing is achieved successively, the respective photosensitive members repeatedly undergo the aforesaid charge, exposure, development, and transfer processes. However, because of a residual electric potential on the photosensitive members after the transfer, the surface of the respective photosensitive members may not be charged uniformly. This residual electric potential affects the formation of a successive electrostatic latent image and causes image defects, such as ghost images.
A recently developed method for solving this problem involves resetting the surface potential of the photosensitive members to 0 V, by radiating a light beam onto the respective photosensitive members before the photosensitive members are charged, and then charging the photosensitive members. For this, an eraser lamp is mounted between the transfer unit and the charge unit and is used to irradiate the photosensitive members. The image quality can be improved by the operation of the eraser lamp.
Besides an eraser lamp, a pre-transfer lamp (PTL), mounted between the developer supply device and the transfer unit, can be used to improve the image quality. The pre-transfer lamp radiates light to the surface of the photosensitive members, to which the developer is adhered, before the transfer is achieved, thereby increasing a transfer efficiency. If the pre-transfer lamp irradiates light to the photosensitive member, an electric potential difference between an exposure area and a non-exposure area, on the surface of the photosensitive member, decreases. Accordingly, the developer adhering to the surface of the photosensitive member can be easily transferred onto the intermediate transfer belt or the printing medium, with a low transfer voltage.
However, the installation of the eraser lamp and the pre-transfer lamp in the conventional image forming apparatus causes an increase in the number of components and in the manufacturing costs.